The Stirling external combustion engine was invented by Robert Stirling over a century and a half ago. But although the engine principles he pioneered have occasionally been incorporated into various prime movers during the ensuing years, Stirling type engines still remain relatively unknown.
Within the past few decades, however, some degree of interest in the Stirling philosophy has been rekindled because a Stirling engine will operate on essentially any heat producing fuel or heat source. And, whereas the by-products of internal combustion engines are difficult to control, external combustion is relatively easy to adjust to minimize harmful by-products. Thus, two major problems in today's world, that of air pollution and the world's dwindling oil reserves, could be to some degree relieved by replacing the widely used internal combustion engine with engines incorporating the principles of Stirling's engine.
Although the basic Stirling configuration is well known in the literature, it will be described next in a cursory manner for the purpose of fixing terms so as to avoid confusion later when comparisons are drawn between it and the improvements of this disclosure. Terms used in the immediately following description are those normally associated with the various components of the basic Stirling engine configuration.
In essence, the Stirling engine comprises six basic parts in its simplest form. These are a main body including a cylinder, a power piston, a displacer piston, a crankshaft and two connecting rods. A heat source is also necessary, but it either may or may not be a part of the engine itself. The cylinder has a closed end and an open end, and the power piston and mating portions of the cylinder are both normally heavy castings machined to close fitting tolerances. Contained entirely inside the cylinder between the power piston and the cylinder's closed end is the displacer piston. Presuming for simplicity that a regenerator is not employed, the displacer piston usually fits quite loosely in the cylinder so that air or other gas also contained in the cylinder is relatively free to flow around it as it reciprocates within its confines. Considerably outboard beyond the open end of the cylinder is the crankshaft. Its rotational axis is perpendicular to, and intersects with, an extension of the cylinder's longitudinal axis. This crankshaft includes two connecting rod cranks angled about 110.degree. apart when viewed down its own centerline or rotational axis. One of the connecting rods connects one crank to the power piston, and the other connecting rod connects the other crank to the displacer piston. But because the power piston is disposed directly between the displacer piston and the crank to which it is connected, the connecting rod for the displacer piston extends through an axial center hole in the power piston. The crank to which the power piston is connected is closer to the crankshaft's rotational axis than is the crank to which the displacer piston is connected. Thus, the displacer piston has a relatively long stroke while the power piston has a relatively short stroke. The approximate 110.degree. angle between crankshaft cranks causes an unusual movement pattern between the two pistons not unlike the action of that toy known as a "yo-yo" in conjunction with the hand of a person operating it.
The operation of the Stirling engine is well known to those skilled in the art, and so a detailed explanation of its operation will not be detailed herein. Suffice it to say that the displacer piston separates the entrapped gas in the cylinder into two variable volume portions or chambers. Presuming again the simplest Stirling configuration without a regenerator, one of these is the expansion space at the cylinder's closed end adjacent the heat source, and the other is the compression space adjacent the power piston. As the gas in the expansion space is heated, the displacer piston moves toward the cylinder's closed end and forces that heated gas around it into the compression space where its now increased pressure bears against the power piston and moves it outwardly which is its power stroke. Expansion of the gas in the compression space accompanies a reduction in its temperature, and as the piston reverses its direction and embarks upon its return stroke, the displacer piston moves to nearly meet it and force the entrapped gas therebetween back around it into the expansion space. On its journey to the expansion space, it is cooled by the cylinder's inner sidewall. These two strokes comprise a complete engine cycle.
Early Stirling designs incorporated very few parts and were thus particularly uncomplicated. However, the transfer of heat from an external source through a solid wall to heat gas in the cylinder did not lend itself to achieving high torque output characteristics, and the squeezing of gas around the displacer piston back and forth did not permit high rotational speeds to be attained, thus holding down power output. Yet, periodically the Stirling engine is re-examined because of the increasing need for engines of inherently non-polluting characteristics capable of burning fuels other than those derived from petroleum. To enhance the engine's poor power and torque output, many modifications to Stirling's basic design concept have been proposed and tested. Usually these modifications complicate the design and increase both in size and weight an engine already considered by many to be too large and heavy for its output.
For example, it has been found that replacing the entrapped air with a more heat conductive gas, such as helium, both increases performance as well as provides output control when the gas pressure can be varied. Unfortunately, increasing the internal gas pressure poses tricky sealing problems. In solving this problem, one experimental group devised a crankshaft arrangement they named "Rhombic Drive" that permitted the two connecting rods to be arranged concentrically and move in a purely reciprocating manner. This is conjunction with a specially designed seal they publicized as a "Roll Sock" that fit over the connecting rods reportedly held the pressurized gas effectively within the cylinder. Other more exotic Stirling engine improvements or variations also have been written up. One of these, the "Rinio", groups four cylinders together and channels the internal gas back and forth between several cylinders. This completely eliminated the displacer pistons, and this "second generation" design has been described as holding great promise for the future of the Stirling concept. Yet, for reasons of efficiency, size, cost, power, torque or whatever, the excitement that accompanied each engineering breakthrough appears to have subsided, and Stirling engines continue in relative obscurity.
It is this background that set the stage for, and utilimately precipitated, the engine improvements to be described in detail hereinafter.